1. Field of the Invention
The present invention relates to a transmission-type or reflection-type liquid crystal display device, or a projection lens for enlarging and projecting display information etc. from a light valve such as DMD, and particularly relates to a projection lens suitable for a rear projection display apparatus such as a rear projection TV, and a projection display apparatus using the projection lens.
2. Description of Related Art
Front projection apparatus and rear projection apparatus have been heretofore known as projection display apparatus. In the front projection apparatus, a projection lens is disposed on the same side as a viewer with respect to a screen so that light outgoing from the projection lens is imaged on the reflection type screen. In the rear projection apparatus, a projection lens and a viewer are disposed to put a screen therebetween so that light outgoing from the projection lens is imaged on the transmission type screen.
Of them, the rear projection display apparatus, for example, like a rear projection television set, has a well known configuration in which constituents between the light source and the screen are put in a cabinet, and light carrying video information is projected from the projection lens disposed on the back face of the cabinet toward the screen disposed on the front face of the cabinet.
In recent years, various projection lenses have been proposed as applications to such a cabinet type projection display apparatus.
An optical system-using a plurality of light valves to obtain a color image requires a synthesis portion for synthesizing light beams from the light valves. It is therefore necessary to secure a certain amount of spacing for a back focal distance of a projection lens (i.e., on the reduction side). Particularly when a projection lens is used in a rear projection TV, the projection lens has to be made wide-angle. Thus, the back focal distance becomes larger than the focal length of the projection lens. In order to miniaturize the apparatus as a whole, it is necessary to insert a reflecting mirror into the projection lens.
Projection lenses disclosed in JP-A-2002-131636, JP-A-2003-287676 and Japanese Patent No. 3244606 etc. are known as wide-angle projection lenses each with a long back focal distance. In any of the documents, there is no consideration about a space to dispose a reflecting mirror in the projection lens. When the reflecting mirror is disposed in the projection lens, the number of lenses will increase so that the manufacturing cost will increase on a large scale.
Projection lenses disclosed in JP-A-2003-156683 and JP-A-2004-354405 are known as solutions to the aforementioned issue.
According to the projection lens disclosed in JP-A-2003-156683 and JP-A-2004-354405, however, an aspheric surface is disposed on the reduction side with respect to an aperture position, and set at a distance from the aperture position, and thus, the balance of the image plane as a whole deteriorates or there survives a difference between the sagittal and tangential planes. Further, the size of the aspheric surface cannot help increasing so as to increase in cost.
According to the projection lens disclosed in JP-A-2003-156683, the power of an aspheric lens itself is made high. Thus, when the aspheric lens is made of plastic, the shape of the lens is deformed excessively due to a temperature change, causing deterioration in performance.
On the other hand, according to the projection lens disclosed in JP-A-2004-354405, the power of an aspheric lens itself is not so high, but the number of lenses is large to be 9 to 11. Radical improvement to reduce such a large number of lenses is required.